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<H1><A NAME="SECTION00620000000000000000">Creating a swap space</A></H1>
<P>
A swap file is an ordinary file; it is in no way special to
the kernel. The only thing that matters to the kernel is that
it has no holes, and that it is prepared for use with
<tt>mkswap</tt><A NAME="1891"> </A>. It must reside on a local disk, however; it
can't reside in a filesystem that has been mounted over NFS
due to implementation reasons.
<P>
The bit about holes is important. The swap file
reserves the disk space so that the kernel can quickly swap out
a page without having to go through all the things
that are necessary when allocating a disk sector to a file.
The kernel merely uses any sectors that have already been
allocated to the file. Because a hole in a file means that
there are no disk sectors allocated (for that place in the
file), it is not good for the kernel to try to use them.
<P>
One good way to create the swap file without holes is through
the following command:
<BLOCKQUOTE> <TT>
<code>$</code> <I>dd if=/dev/zero of=/extra-swap bs=1024 count=1024</I><BR>
<code>1024+0 records in</code> <BR>
<code>1024+0 records out</code> <BR>
<code>$</code>
</TT></BLOCKQUOTE>
where <tt>/extra-swap</tt><A NAME="1893"> </A> is the name of the swap file and
the size of is given after the <tt>count=</tt>. It is best for
the size to be a multiple of 4, because the kernel writes out
<b>memory pages</b>, which are 4 kilobytes in size. If the
size is not a multiple of 4, the last couple of kilobytes may
be unused.
<P>
A swap partition is also not special in any way. You create it
just like any other partition; the only difference is that it is
used as a raw partition, that is, it will not contain any
filesystem at all. It is a good idea to mark swap partitions as
type 82 (Linux swap); this will the make partition listings
clearer, even though it is not strictly necessary to the kernel.
<P>
After you have created a swap file or a swap partition, you
need to write a signature to its beginning; this contains some
administrative information and is used by the kernel. The
command to do this is <tt>mkswap</tt><A NAME="1896"> </A>, used like this:
<BLOCKQUOTE> <TT>
<code>$</code> <I>mkswap /extra-swap 1024</I> <BR>
<code>Setting up swapspace, size = 1044480 bytes</code> <BR>
<code>$</code>
</TT></BLOCKQUOTE>
Note that the swap space is still not in use yet: it
exists, but the kernel does not use it to provide virtual
memory.
<P>
You should be very careful when using <tt>mkswap</tt><A NAME="1898"> </A>, since it
does not check that the file or partition isn't used for anything
else. <em>You can easily overwrite important files and
partitions with <tt>mkswap</tt><A NAME="1900"> </A>!</em> Fortunately, you should only
need to use <tt>mkswap</tt><A NAME="1902"> </A> when you install your system.
<P>
The Linux memory manager limits the size of each swap space to
about 127 MB (for various technical reasons, the actual limit
is <IMG WIDTH=78 HEIGHT=12 ALIGN=MIDDLE ALT="tex2html_wrap_inline4269" SRC="img11.gif"> bytes, or
127.6875 megabytes). You can, however, use up to
16 swap spaces simultaneously, for a total of almost
2 GB.<A NAME="tex2html27" HREF="footnode.html#1836"><IMG ALIGN=BOTTOM ALT="gif" SRC="./foot_motif.gif"></A>
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<P><ADDRESS>
<I>Lars Wirzenius <BR>
Sat Nov 15 02:32:11 EET 1997</I>
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